1. Field of the Invention
The present invention generally relates to microsensors for assaying analytes. In particular, the present invention relates to microcantilevers and methods of making and using thereof.
2. Description of the Related Art
The rapid growth of nanotechnology has led to new horizons for the development of microsensors that can be used to detect, measure, analyze, and monitor chemical and biological agents in samples of a few microliters or less. Microsensors can be used to detect, measure, analyze, or monitor hazardous biological and chemical agents such as organisms belonging to Bacillus, Mycobacterium, Francisella, Brucella, Clostridium, Yersinia, Variola, Orthopox, and Burkholderia, toxins, chemical warfare agents, organophosphates (OPs), pesticides, insecticides, and the like in an environment. Microsensors may also be used in clinical settings to screen a patient for the presence of a disease, determine a patient's likelihood of obtaining a given disease, determine a patient's susceptibility to a given drug, and the like.
Microsensors include microfluidic devices such as biochips which comprise a collection of microarrays such as DNA or protein microarrays arranged on a solid support. An example of a biochip is the biological IC chip. See Vo-Dinh, T. et al. (2001) Nanosensors and Biochips: Frontiers in Biomolecular Diagnostics, Sensors and Actuators B 74:1560-1564.
Microsensors containing microcantilevers have been shown to be sensitive and accurate. See Wu, G., et al. (2001) Nature Biotech. 19 (9):856-860. Changes in the physical properties of a microcantilever is used to detect changes in the environment of the microcantilever. Most often the deflection or conductivity of the microcantilever is measured and then used to indicate the presence or absence of a certain analyte. Microcantilevers are commonly made of silicon, silicon nitride, glass, metal, or combinations thereof.
For use in assays for biological or chemical agents, the microcantilevers are commonly a bimaterial, such as gold on one side and silicon on the other side. The gold side is then coated with a receptor that specifically binds a given ligand. Receptor/ligand pairs include antibodies and antigens, complementary nucleotide sequences, receptors and small molecules, and the like. See Raiteri, R., et al. (1999) Sensors and Actuators B 61:213-217. When the receptor binds the ligand, the side coated with the receptor will either become tensioned or relieved, thereby causing the microcantilever to deflect. The concentration of the ligand can be determined by the degree of deflection. The amount of deflection is usually in nanometer lengths that may be measured using various techniques known in the art such as optical techniques.
Unfortunately, microcantilevers are subject to turbulence in the liquid flow of samples which affects the accuracy of the measurements. See Raiteri, R., et al. (1999) Sensors and Actuators B 61:213-217, and Fritz, J., et al. (2000) Science 288:316-318. Additionally, microcantilevers in the prior art have low sensitivity especially for low analyte concentration and variations in sample temperature can produce unwanted deflections due to bimaterial effects as discussed by Fritz, J. et al. (2000) Science 288:316-318.
Conventional (non-piezoresistive) microcantilevers used along with an optical detection systems require rigorous alignment of the detecting laser beam with respect to the microcantilever position. In some cases, the laser beam is aligned to hit a shiny silicon surface or a metal coated sensing area on the back of the microcantilever. In a liquid environment, turbulence effects may result in additional three-dimensional deflections of the microcantilever beam which will render any detection measurements useless. In addition, the presence of a focused laser beam in a liquid environment can result in additional thermal effects that can result in extraneous readings.
Therefore, a need exists for microcantilevers that are highly sensitive and have low turbulence effects.